1. Field of the Invention
This invention relates to ethylene copolymers and more specifically, it relates to blends of ethylene copolymers at least one of which is crosslinked and adhesives based thereon.
2. Description of the Prior Art
Ethylene/vinyl acetate (E/VA), and, to a considerably lesser extent, ethylene/ethyl acrylate (E/EA) copolymers have been considered for use in pressure sensitive adhesives for more than 13 years ("Technical Information Letter #3842," Aug. 1, 1966, Pennsylvania Industrial Chemical Corp.). This reference from the Pennsylvania Industrial Chemical Corporation presents data on blends of Elvax.RTM. 40 (E/VA, 40 wt % VA, 57 MI) with various "Picco" hydrocarbon tackifying resins. Of six resins tested the data indicate that "Piccovar" L60 was the best of the group and that an Elvax.RTM. 40/"Piccovar" L60 ratio of 40/60 gave better performance than 50/50 or 60/40 ratios. The other tackifying resins in this program were:
"Picco" 25 PA1 "Piccovar" L30 PA1 "Piccolastic" A25 PA1 "Piccolastic" A50 PA1 "Piccolastic" A75 PA1 Rosins PA1 Esters of hydrogenated rosins PA1 Esters of polymerized rosins PA1 Terpene hydrocarbon resins PA1 Aliphatic hydrocarbon resins PA1 Aromatic hydrocarbon resins PA1 Terpene phenolic resins PA1 Styrene-based resins PA1 (a) from about 14 to about 60 percent by weight of a homogeneous blend of at least one ethylene copolymer (A) and at least one ethylene copolymer (B), each copolymer having at least one polar comonomer selected from the group consisting of vinyl esters of saturated carboxylic acids wherein the acid moiety has up to 4 carbon atoms, unsaturated mono- or dicarboxylic acids of 3 to 5 carbon atoms, and esters of said unsaturated mono- or dicarboxylic acids wherein the alcohol moiety has 1 to 8 carbon atoms, copolymer (A) having a minimum polar comonomer content that is sufficient to yield the desired tack in pressure sensitive adhesive formulations and having a maximum polar comonomer content that will still maintain the desired bleed resistance when used in the formulation of this claim and a melt index of from about 3 to about 200, and copolymer (B) having a polar comonomer content of from about 10 to about 33 percent by weight, an ethylene content of from about 67 to about 90 percent by weight and a melt index of from about 1 to about 100, provided that the polar comonomer content of copolymer (A) is at least 5 percent by weight higher than the polar comonomer content of copolymer (B), copolymer(s) (A) being present in an amount of from about 10 to about 90 percent based upon the weight of total copolymer and copolymer(s) (B) being present in an amount of from about 10 to about 90 percent based upon the weight of total copolymer, said homogeneous blend of copolymers (A) and (B) having been mildly cross-linked to reduce the melt index of said blend by a factor of from about 2 to about 150; PA1 (b) from 0 to about 25 percent by weight of filler; PA1 (c) from 0 to about 5 percent by weight of antioxidant; PA1 (d) from about 40 to about 86 percent by weight of tackifier; and PA1 (e) from about 0 to about 25 percent by weight of plasticizer. PA1 (a) from about 14 to about 60 percent by weight of a mixture of at least one ethylene copolymer (A) and at least one ethylene copolymer (B), each copolymer having at least one polar comonomer selected from the group consisting of vinyl esters of saturated carboxylic acids wherein the acid moiety has up to 4 carbon atoms, unsaturated mono- or dicarboxylic acids of 3 to 5 carbon atoms, and esters of said unsaturated mono- or dicarboxylic acids wherein the alcohol moiety has 1 to 8 carbon atoms, copolymer (A) having a minimum polar comonomer content that is sufficient to yield the desired tack in pressure sensitive adhesive formulations and having a maximum polar comonomer content that will still maintain the desired bleed resistance when used in the formulation of this claim and a melt index of from about 3 to about 200, and copolymer (B) having a polar comonomer content of from about 10 to about 33 percent by weight, an ethylene content of from about 67 to about 90 percent by weight and a melt index of from about 1 to about 100; provided that the polar comonomer content of copolymer (A) is at least 5 percent by weight higher than the polar comonomer content of copolymer (B), copolymer(s) (A) being present in an amount of from about 10 to about 90 percent based upon the weight of total copolymer and copolymer(s) (B) being present in an amount of from about 10 to about 90 percent based upon the weight of total copolymer, at least one copolymer of said copolymer(s) (A) and copolymer(s) (B) having been mildly crosslinked to reduce its melt index by a factor of from about 2 to about 200 for copolymer(s) (A) and by a factor of from about 2 to about 100 for copolymer(s) (B) such that at least 15 percent of copolymer, based upon the weight of total copolymer, is crosslinked; PA1 (b) from 0 to about 25 percent by weight of filler; PA1 (c) from 0 to about 5 percent by weight of antioxidant; PA1 (d) from about 39 to about 85 percent by weight of tackifier; and PA1 (e) from about 0.5 to about 25 percent by weight of plasticizer. PA1 a. Wood, gum or tall oil derived rosin acids. They can be hydrogenated, disproportionated or mildly polymerized. PA1 b. Esters of the various classes of rosin acids referred to above. Alcohols suitable for preparing these esters include mono- and polyethylene glycols, glycerol, pentaerythritol and related products. PA1 c. Terpene resins. PA1 d. Dicyclopentadiene-aromatic hydrocarbon resins such as those described in U.S. Pat. No. 3,023,200 covering "Piccovar" resin manufactured by Pennsylvania Industrial Chemical Co. PA1 e. Low molecular weight resins based on styrene and/or substituted styrenes. PA1 a. Typical phthalates, azelates, adipates, tricresyl phosphate, and polyesters, such as those used in flexibilizing polyvinyl chloride. PA1 b. Low molecular weight resins made from alkylated phenols, phenol modified coumaroneindene, terpenes and synthetic terpenes. PA1 c. Petroleum derived processing oils. These oils are widely used in the compounding and/or extending of rubber compositions. They are generally classed as aromatic, naphthenic or aliphatic in character, the classification just identifying the major oil type present. They are available over a wide range of viscosities, from as low as 60, to over 15,000 SUS viscosity at 100.degree. F. (37.8.degree. C.). The type of oil and its viscosity both have some effect on the performance of the HMPSA in which it is used. Generally speaking, oils with a viscosity of greater than 200 SUS at 100.degree. F. (37.8.degree. C.) are of most use in the compositions of this invention. PA1 a. High energy radiation as described in U.S. Pat. No. 3,734,843. PA1 b. Peroxide generated free radicals as described in U.S. Pat. No. 3,988,509. PA1 c. Mechanically working the polymer below its degradation temperature as described in U.S. Pat. No. 3,968,091. PA1 d. UV light radiation, etc. PA1 (1) Adhesive blend preparation PA1 (2) Coating the adhesive onto test substrates PA1 (3) Adhesive testing PA1 "Polyken" Tack PA1 Shear Hold time at 23.degree. C.
Formulations based on E/VA copolymers are covered in three articles by R. J. Litz of Union Carbide Corp. ("EVA Systems for Hot Melt Pressure-Sensitive Adhesives," R. J. Litz, Adhesives Age, August 1971, pages 32-4; "Hot Melt Pressure-Sensitive Adhesives Based on EVA Copolymers," R. J. Litz, TAPPI Vol. 57, pages 84-86, June 1974; "Developments in Ethylene-Based Hot Melt Adhesives," R. J. Litz, Adhesives Age, August 1974, pages 35-38). In these publications formulations are given which contain:
______________________________________ Parts by Weight ______________________________________ 1. Polymer 35-50 2. Tackifier 30-50 3. Stabilizer 0.1-0.5 4. Plasticizer 0-20 5. Filler 0-5 ______________________________________
The use of two different EVA copolymers (neither of which was crosslinked) with different vinyl acetate contents and melt indices is described in the June 1974 publication. One formulation contained:
______________________________________ Parts by Wt. VA, Wt. % Melt Index ______________________________________ Co-Mer EVA-501 15 28 350 Co-Mer EVA-605 30 33 20 ______________________________________
The use of copolymer blends of different melt index was stressed more than the use of copolymers with different VA contents. Copolymers with VA contents of over 33% were not mentioned.
Rifi of Union Carbide Corp. ("Pressure Sensitive Adhesives-Structure-Performance Relationship," M. R. Rifi, presented at the 1976 Fall Seminar of the Adhesive & Sealant Council; "Pressure Sensitive Adhesives-Structure vs Performance," M. R. Rifi, presented at the TAPPI Western Hot Melt Meeting, November 1977) discussed hot melt pressure sensitive adhesives based on E/EA copolymers. A series of three compositions containing 35, 42.5 and 50 wt % of Carbide's "DPDA" 2304 all used "Piccovar" L60 as the tackifier. "DPDA" 2304 is an E/EA polymer with a 20 MI and containing 23 wt % EA. Rifi also discussed the performance of E/VA-based pressure sensitive adhesives using EVA-605 and EVA-501, as well as those using blends of 605 with 501 and blends of E/VA with E/EA polymers. None of these copolymers were crosslinked.
The formulation work described by Litz and Rifi primarily was based on 28 and 33 wt % VA copolymers. However, Duncan and Bergerhouse of U.S. Industrial Chemicals Co. ("EVA and VAE Copolymers for Hot Melt PSA's," R. E. Duncan and J. E. Bergerhouse, Adhesives Age, March 1980, pages 37-41) reported data on a much more comprehensive study of hot melt pressure sensitive adhesive formulation. This study included E/VA copolymers containing 18, 28, 33, 40, 50 and 60 wt % VA blended in 3 different ratios with 50 representative tackifying resins of the classes:
In none of these papers published by Carbide and USI on HMPSA based on E/VA and E/EA copolymers has the very serious problem of adhesive bleed into the label stock or into the substrate to which it is applied been mentioned. Poor performance in bleed tests of formulations such as they discuss has kept their use in the HMPSA label market at a very low level.
Meron and Skeist ("Trends in Pressure-Sensitive and Heat Seal Materials," J. Miron and I. Skeist, Adhesives Age, pages 35-38, January 1978) state "The first efforts to make pressure sensitive hot melts concentrated on tackifying and plasticizing EVA, ethyl cellulose or acrylics but the products obtained showed poor cohesion and excessive cold flow." Maletsky and Villa ("Pressure Sensitive Hot Melt Adhesives-Background, Current and Future Potential," A. Maletsky and G. J. Villa, presented at the Pressure Sensitive Tape Council Meeting, Washington, D.C., March 5, 1975) came to the same conclusion about E/VA based HMPSA stating they showed too much cold flow at 120.degree.-140.degree. F. to be of general utility.
Two interesting articles ("Hot Melt Application and End Use Requisites: Their Characterization and Measurements," J. A. Collins, presented at the TAPPI HMPSA Short Course, Boxborough, Mass., May 5, 1976 and "Evaluation and Selection of Pressure Sensitive Hot Melt Adhesives" C. Watson, presented at the TAPPI 1977 Cavalcade of Hot Melts) have been published in the last several years concerning the evaluation and selection of HMPSA. Both authors mention the importance of bleed tests at 140.degree. F. in determining the merit of HMPSA. Collins states:
"For manufacturers of products that coat PSA HM's on porous substrates such as paper a bleeding test should be used. PSA's that do not age well may bleed or stain the porous substrates. This becomes especially critical when the coated substrate is wound tightly in a web. This test is important because, when the adhesive penetrates, it may leave less effective film thicknesses and discolor the face stock. The result can be a drop in tack, peel strength, quick stick and aesthetic appearance. This test is usually run by exposing a given area, usually a 2 sq. in. section to a given temperature, usually 140.degree. F., for 14 days. These conditions can be altered to fit the needs of a specific end user. After the aging period the samples should be evaluated for color change, peel adhesion, shear adhesion, quick stick and tack. This is done by comparing the tested sample with a control coated sample."
B. H. Gregory, USI Europe N.V. ("A Review of the Status and Development of EVA Copolymers," B. H. Gregory, presented at the 1979 TAPPI Hot Melt Conference at Antwerp, Belgium) presented a paper at the 1979 TAPPI Antwerp Conference on Hot Melts in which he compared blending two or more E/VA copolymers (none of which were crosslinked) of suitable MI and VA levels so as to match the MI and VA level of a single, directly synthesized, polymer. He says that by blending of any two or three grades in the range of about 10 to 40% VA and 3 to 600 MI the mechanical and thermal properties of the copolymer blend essentially match those of a single copolymer prepared by direct synthesis to match the MI and VA content of the blend.
Many patents, both U.S. and foreign, have been issued covering E/VA and E/acrylates in pressure sensitive adhesive formulations.
U.S. Pat. No. 3,342,902 discloses a hot melt adhesive having pressure sensitivity is a blend of atactic polypropylene, an E/VA copolymer and a polyterpene resin.
U.S. Pat. No. 3,445,263 discloses PSA tapes prepared by hot melt coating high density polyethylene with an adhesive based on an E/VA copolymer and hydrogenated wood rosin esters.
U.S. Pat. No. 3,515,270 discloses HMPSA formulation that includes an E/VA copolymer and a low molecular weight polystyrene resin.
U.S. Pat. No. 3,630,980 discloses a HMPSA that includes an E/VA copolymer, a resinous rubbery block copolymer of styrene with butadiene or isoprene, and a third component which may be a modified or unmodified rosin, a coumarone/indene resin, a polyterpene resin, a diene-olefin hydrocarbon resin or a polystyrene.
U.S. Pat. No. 3,644,252 discloses a HMPSA composition that includes a primary polymer which is a random copolymer of styrene and isobutylene and a secondary polymer which may be an E/VA copolymer, an E/alkyl acrylate copolymer, a polyvinyl alkyl ether or a terpolymer of E/VA/AA or E/VA/MAA.
U.S. Pat. No. 3,792,002 discloses a HMPSA composition that includes a polyamide resin, a tackifier, a second polyamide or polyester amide and/or an E/VA copolymer.
U.S. Pat. No. 3,849,353 discloses a HMPSA that includes a copolymer of E/VA and/or an alkyl acrylate, atactic polypropylene, and a vulcanized rubber such as butyl rubber.
Canadian 845,755 discloses a HMPSA that includes a resinous E/VA copolymer, a modified or unmodified rosin, a coumarone-indene resin or a polystyrene resin.